All batteries involve a comparison between two chemical reactions that take place at different places within the battery. The negative end of the battery is connected to a metal that is releasing electrons (in the case of the battery we made, zinc metal is turning into positively charged zinc ions by giving up two electrons for each zinc atom: Zn --> Zn++ + 2 e-). At the positive end, electrons are combining with a different positive ion (for example, two hydrogen ions plus two electrons makes one molecule of hydrogen gas: 2 H+ + 2 e- --> H2). It costs energy to remove the electrons at one side (i.e. from the zinc) but we gain more energy at the other. The net release of energy is what makes the current go around the circuit. By having the two reactions separated from each other, they cannot take place except when the current is flowing.
The voltage of the battery is determined by the amount of
energy released in the two chemical reactions, and is largely
determined by the chemical reactions that are involved.
Our battery would work much better if we had a better chemical
reaction at the positive end. For example, if there were copper
ions in the solution, they could absorb the electrons (producing
copper metal). Another possibility is to add hydrogen peroxide
to the solution, allowing the reaction
H2O2 + 2 H+ + 2 e- --> 2 H2O
-- that is, hydrogen peroxide + hydrogen ions + electrons makes water.
Using magnesium metal instead of zinc metal also makes a better battery, because it costs less energy to remove the electron from magnesium metal (this means it is more chemically reactive). Theoretically, aluminum would be a good choice, but in practice it doesn't work very well, because the aluminum surface is almost completely covered by a thin layer of aluminum oxide, which is an insulator: aluminum is too reactive to use in air.
The current flow is limited by the rate at which the chemical reactions can take place. The reactants in contact with the conducting surface get used up quickly, and then nothing more can happen until more chemical moves into place. This is why the LED starts bright but quickly dims when we connect it to the battery we made. It is also why this battery will not light a light bulb.
We have found no design using common chemicals that will cause a light bulb to glow. The voltage (measured when the battery is not doing anything) is not high enough, and a set of the batteries in series will not work either, because the chemical reactions take place too slowly, which limits the amount of power that the battery can deliver. We are surprised that published descriptions of ways to make a battery frequently invite you to try to light a light bulb with it -- evidently the authors have never tried their own experiment.
In producing the design described in this section we have tried a number of designs. Here is a brief review
copper metal | copper sulfate solution | salt solution | zinc metal
At the copper surface, the copper ions in the copper sulfate solution are turning into copper metal. This is easier than trying to make hydrogen gas, and gives a higher voltage. However, the copper solution must be isolated from the zinc surface, or it would directly react with it, so we need another layer of a solution that does not contain copper (it eventually will become a solution of zinc sulfate). Making this kind of battery requires having two paper layers, one wetted with copper sulfate and one wetted with salt solution. Since copper solutions are rather poisonous, we didn't try to develop this battery.
The section on the sources of electrical energy